1. Field of the Invention
The present invention relates to an electrophotographic developer that can be used in an electrophotographic apparatus utilizing an electrophotographic process, such as a duplicator, a printer and a facsimile machine, and a process for forming an image using the electrophotographic developer.
2. Description of the Related Art
Conventional examples of a fixing process of a toner for developing an electrostatic image include a pressure fixing process by using only a pressure roll at an ordinary temperature and a contact heat fixing process using a heating roll, and also an oven fixing process using heating with an oven, a flash fixing process using a xenon lamp, an electromagnetic wave fixing process using microwaves, and a non-contact fixing process, such as a solvent fixing process using vapor of a solvent, and processes using heat, such as an oven fixing process and a contact heat fixing process, are mainly employed from the standpoint of reliability and safety. In particular, the contact heating process using a heating roll or belt is constituted from a heating roll or a heating belt having a heating unit, and a pressure roll or a pressure belt. Fixing is effected by passing a fixing sheet (transfer material) with the toner image surface in contact with the surface of the heating roll or the heating belt under pressure. The process has such characteristics that the fixing can be effected with high heat efficiency at a high rate since the surface of the heating roll or the heating belt and the toner image surface of the fixing sheet are in direct contact with each other, and has been widely employed owing to the characteristics.
In the heat fixing process, shortening of the so-called warm up time, i.e., the time from turning on the power to the fixing device reaching the usable temperature to enable fixing, and reduction in fixing temperature for saving the consumption of energy are demanded. In particular, it is demanded in recent years that electrification to the fixing device is terminated except for the time operation for reducing the consumption of energy, and therefore, fixing at further lower temperature is demanded to reach the temperature of the fixing device to the fixable temperature at once. Furthermore, decrease in fixing temperature is preferred from the standpoint of costs because the printing rage can be increased by decreasing the fixing temperature with the same electric power consumption, and the service life of fixing members, such as a heating roll, in the contact heat fixing process can be prolonged.
However, decrease of the fixing temperature of the toner in the conventional processes brings about decrease of the glass transition point of the toner particles, and therefore the storage stability of the toner cannot be simultaneously established. In order to ensure both the low temperature fixing and the storage stability of the toner at the same time, it is necessary to endow the toner with the so-called sharp melt property, i.e., the viscosity of the toner is quickly decreased in a high temperature region with the glass transition point of the toner maintained at a high temperature.
However, since the resin used in the toner generally has certain widths in glass transition point and molecular weight, it is necessary to endow the sharp melt property that the composition and the molecular weight of the resin are aligned to an extreme extent. In order to obtain such resins, it is necessary to align the molecular weight of the resin by employing a special production process or by treating the resin with chromatography, and in such cases, the production cost of the resin in necessarily increased, and waste resins occur upon production. Therefore, it is not preferred from the standpoint of resource saving and environmental protection in recent years.
It has been studied to realize the low temperature fixing property that a crystalline resin is used as the binder resin of the toner (JP-B-56-13943, JP-B-62-39428, JP-B-63-25335, JP-B-62-39428, JP-B-4-30014, JP-A-4-120554, JP-A-4-239021 and JP-A-5-165252). When a crystalline resin is used, the hardness of the toner is maintained at a temperature lower than the melting point of the crystal, and the viscosity of the toner is suddenly decreased at a temperature exceeding the melting point owing to melting of the crystal, so as to realize the low temperature fixing.
However, the crystalline resin is difficult to retain electric charge in comparison to the conventional amorphous resins, and therefore, in the case where the crystalline resin as the binder resin of the toner, it is difficult to impart sufficient charge amount to the toner.
The invention has been made in view of the foregoing circumstances and is to solve the problems associated with the conventional art. The invention is to provide an electrophotographic developer and a process for forming an image in that the fixing can be conducted at a lower temperature than the conventional technique, whereby the energy consumption on the fixing step is greatly decreased, and the warm up tome is shortened.
The invention is to also provide an electrophotographic developer and a process for forming an image that are excellent in the powder storage stability, the anti-blocking property and the image storage stability after fixing without impairing the density, the coloring property and the transparency of the image after fixing.
The invention is to further provide an electrophotographic developer and a process for forming an image having appropriate electric charging property without any problem of fogging or cloud and being excellent in maintenance property of electric charge.
According to an aspect, the invention relates to an electrophotographic developer containing a toner for developing an electrostatic image and a carrier. The toner contains a colorant and a binder resin that contains a crystalline resin having a melting point of about from 60xc2x0 C. to 120xc2x0 C. as a main component. The carrier has a resin coating containing a nitrogen-containing resin.
According to the invention, fixing can be attained at a low temperature by using the crystalline resin having an appropriate melting point as the main component of the binder resin, so as to provide an electrophotographic developer excellent in the powder storage stability, the anti-blocking property and the image storage stability after fixing. Furthermore, a carrier having the resin film containing a nitrogen-containing resin on the surface thereof is combined with the crystalline resin, which is generally short in charging property, so as to remarkably improve the charging property, whereby the electrophotographic developer of the invention has good charging property as an entire electrophotographic developer.
In the electrophotographic developer of the invention, the carrier preferably has a nitrogen content on a surface thereof of about from 0.1 to 50% by atom, and the resin film on the surface of the carrier preferably contains at least one of a urea resin, a urethane resin, a melamine resin and an amide resin.
In the electrophotographic developer of the invention, the crystalline resin in the toner is preferably a crystalline polyester, and more preferably an aliphatic crystalline polyester. In the case where the crystalline polyester is used as the crystalline resin, it is preferred that the crystalline polyester is synthesized from an acid component and an alcohol component, with at least one of the acid component and the alcohol component containing a component containing a sulfonic acid group.
According to another aspect, the invention relates to a process for forming an image containing the steps of: forming an electrostatic latent image on a latent image bearing member; developing the electrostatic latent image formed on the latent image bearing member with a developer carried on a developer bearing member to form a toner image; transferring the toner image formed on the latent image bearing member to a transfer material; and fixing the toner image transferred to the transfer material with heat, the developer being the electrophotographic developer according to the invention.